The thing to look for is a beacon. Of course, you’ll only see one if the person who claimed the base followed the proper procedure, and placed a beacon within their base boundary.
But the thing is, the ONLY time you’ll see another player’s beacon is when it’s inside their base. So, if you warp into a new system, and you see a beacon, and you didn’t put it there, then you KNOW it’s an occupied base.
And unlike some other symbols or markers, another player’s beacon within their base is always visible when you scan a new system. Scan, find a beacon, and you know you have an occupied base.
Comm balls are different. They generally don’t show up until you land on a planet. Once you do, they will be visible from space afterwards - but you have to land on the right planet first.
As @Mad-Hatter said, the “contact” sign just means that a system has been claimed by someone else - it doesn’t necessarily mean they have a base there. But it does mean that you can rule out systems that don’t have the “contact” marker.
You can’t build a base in a system you haven’t registered - claiming the base automatically registers the system to you. So whilst not all “contact” systems will have a base, all registered bases are in “contact” systems.
It’s pointless looking for bases in non-“contact” systems - there are none. If you’re looking for bases, only look in “contact” systems. If you scan the system, and get a message “XXXX base found”, that’s great. But if you don’t, that doesn’t mean there’s no base there.
If you scan the system, and you see a beacon, and you didn’t place it, then that’s a base. Go to it.
If you don’t see a beacon, then land on each of the planets, exit your ship, and look around. That’s the only way you can be sure of seeing any comm balls.
If you do see a comm ball, visit it, and find out what it is. Most of the time, it’ll be nothing. But every now and then, it will be an occupied base.
Best answer I can give. The game’s not perfect - and nor am I.